In the ad, called "Built to Serve," King's words from a speech from 50 years ago play over images including scenes of firemen at work, volunteers handing out turkeys, rescue workers on the job and a member of the military returning home to embrace his child.

Missing from the ad are the slain civil rights leader's words in the same speech guarding against commercialism: "In order to make your neighbors envious, you must drive this type of car ... And you know, before you know it, you're just buying that stuff. That's the way advertisers do it."

On Twitter, most people expressed the idea that using King's speech to "sell trucks" crossed a line.

"They pushed it over the edge," said Kelly O'Keefe, a professor at Virginia Commonwealth University's Brandcenter. "You wanted to root for it because the cause is good, but it just didn't end up fitting the brand, so you ended up feeling a little bit manipulated."

Fiat Chrysler said in a statement that it worked closely with the King estate on the ad.

The firm managing King's intellectual property, Intellectual Properties Management, said in a statement that it approved the ad because it embodied King's philosophy.

The King Center in Atlanta said neither it nor Bernice King, King's daughter, are in charge of licensing King's speeches or other intellectual property.

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